How to move the handles of a sequence via scripting?
You can reference the sequence object immediately when creating it:

seq = bpy.context.scene.sequence_editor.sequences.new_image("coolName", "somePath/img.PNG", 4, 1)


Then you can edit the frame_final_start and frame_final_end to edit the sequence start and finish, same as the handles:

seq.frame_final_start = 10
seq.frame_final_end   = 50